Electromagnetic relay circuit



May 27, 1947. M HADFlELD 2,421,148

ELECTROMAGNETIC, RELAY CIRCUIT Filed May 22, 1943 FiGi ae. IA

INVENTOR. BERTRAM MORTON HADFIELD T ATTOR N EY- Patented May 27, 1947UNITED STATES PATENT OFFICE ELECTROMAGNETIC RELAY CIRCUIT wareApplication May 22, 1943, Serial No. 488,026 In Great Britain June 12,1942 Ciaims.

The present invention concerns improvements in or relating toelectromagnetic relay circuits and has for its object the improvement ofthe circuit conditions under which electromagnetic relays function, withparticular reference to the case where the relay is intended to releaseslowly, and without detriment to the normal require ments as regardsampere-turns for operation or maximum release lag after operation. Afurther object is to enable the steady state flux to be obtained in aperiod of time shorter than that required to operate other magnetsystems commonly used in automatic telephony, so that the breakdown ofsuch systems becomes only dependent on the operate and release times ofsuch other magnets.

According to one feature of the invention, the buildup of flux in therelay on application of the battery is made dependent on a circuitcomprising the relay and a series resistance, whilst the decay of fluxin the relay on removal of the battery is made dependent on a circuitcomprising the relay and a shunt resistance. The series resistance maybe permanently connected in series with the relay and the operatingcontact and battery, whilst the shunt resistance may be connected to therelay via a non-linear device which only becomes effective in completingthe shunt circuit when the operating contact opens.

According to a further feature of the invention, the steady stateampere-turns or flux in the relay is maintained at normal values byreducing the number of turns on the relay and increasing the currentfrom the battery, by comparison with a relay operated directly from thebattery, without necessarily increasing the current density in the wireor the permissible wattage dissipation of the relay.

According to a further feature of the invention, the time for the fluxto attain the steady state upon closure of the relay circuit is madesubstantially equal to the time constant of the relay circuit includingthe series resistance, by shunting the latter with a suitable condenser.The value of the condenser may be such as to give a minimum of overshooton the steady state flux, or may be such as to give a greater overshootwith the object of ensuring that magnetic saturation always takes placeeven when the closure time of the circuit is as short as the above timeconstant, so that the release-flux-time-decay curve is at leastmaintained. In this manner the static release lag is maintained for allcircuit closure times down to that which is necessary to produce thesteady state flux, and which time as has been stated, is substantiallyequal to the time constant of the relay circuit including the seriesresistance.

The invention will be better understood by reierring to the accompanyingdrawing, in which:

Fig. 1 is a schematic diagram illustrating the use of a series resistorand a shunt rectifier to control the waveform of current flowing throughthe holding relay of an impulsing circuit,

Fig. 1A is a modification of Fig. 1 wherein the shunt rectifier isreplaced by additional contacts on the impulsing relay,

Fig. 2 is another modification of Fig. 1 wherein a condenser is bridgedacross the series resistor to accelerate the build-up of current in theholding relay during impulsing,

Fig. 3 is an extension of Fig. 2 illustrating the use of an additionalrectifier to enable the holding relay to be held over another circuitwithout affecting the operation of a magnet controlled by the impulsingcontacts, and

Fig. 4 illustrates the application of the foregoing basic circuits to atwo-motion stepping switch.

In order that a better appreciation of the invention may be obtained itis first necessary to establish the essential conditions for ensuringthat the normal operating ampere-turns and release time may be obtained,and secondly to establish the conditions for ensuring a substantialreduction in operate time without conflicting with the former.

It can be shown that the release time of a relay with a short-circuitedwinding, such as a slugged relay, where the release is initiated by adisconnection of the battery from the operating coil, is due entirely tothe time constant of the short-circuited winding. It can also be shownthat the time constant of a winding on a relay of a given magneticcircuit, is proportional to the Wound area divided by the product of thespecific resistance of the metal and the length of the mean turn (bywound area is meant the total cross-sectional area of the winding,exclusive of insulation or air space). If the latter product be taken asconstant for both operating and short-circuited windings, then the timeconstants of such windings are proportional to the wound areas. Hencethe release time of a relay having an operating winding and ashort-circuited winding or slug is proportional to the wound area of thelatter.

The winding space factor for enamel wire is approximately 0.6 over awide range of commonly used gauges, and hence a coil fully wound withsuch wire will have a time constant equal to 0.6

that of a solid slug of the same metal as the wire and having the sameshape and total volume. As the slug, or a short-circuited winding, cannever fill all the available winding space and in fact in a typicalrelay at least 0.4 of the space must be left for the operating winding,it follows that the release time of a fully wound relay whenshort-circuited is at least equal to that of a normal slugged relay andis greater than that of an auxiliary relay with a short-circuited winching. Hence if the relay be fully wound and effectively short-circuitedon release, the release time will be at least as long as for a normalslugged relay, and independent of the number of turns or resistance ofthe relay provided the normal operating ampere-turns are obtained.

As regards the steady state ampere-turns, it can be shown that they areequal to the product of the current density in the wire and the woundarea. normal for a given class of relay, the normal ampere-turns can beobtained without any increase in current density, and independently ofthe number of turns or resistance.

As regards the permissible wattage dissipation of the relay it can beshown that the watts are equal to the product of the square of theampereturns and the resistance of a single turn winding filling thewound area. The wattage dissipation will therefore be normal if thewound area is normal and the ampere-turns are normal, and is independentof the number of turns or resistance.

To sum up therefore, it can be seen that provided the relay is woundnormally, and preferably fully, then the normal ampere-turns and timeGUEST/all! can be obtained with normal requirements as to currentdensity and dissipation and independently of factors such as steadystate current or voltage or number of turns or resistance. It followsthat the manner in which the necessary ampere-turns are obtained onoperation will not 'aiiect the normal requirements during the steadystate period of operation nor the release lags obtainable byshort-circuiting on release.

It can be shown that theoperate time of a relay is proportional to theproduct of the ampere-turns and number of turns divided by the batteryvoltage. For a given, normal value of ampere turns, therefore, theoperate time can be reduced by increasing the battery voltage orreducing the number of turns, or both. Generally speaking the batteryvoltage is fixed by other considerations, and in what follows it will beassumed that only the turns may be reduced.

It can be shown that the number of turns is equal to the steady voltagedrop on the relay divided by the product of the ampere-turns md theresistance of a single turn occupying the wound area. Hence for a givenwound area and given ampere-turns, the voltage drop is proportional tothe number of turns and if the latter are reduced then the differencevoltage must be absorbed by a series resistance.

It can therefore be stated that the operate time of a relay with a givenwound area and ampereturns, can be reduced by a series resistance andThus, provided the wound area remains I in the same ratio as theresulting voltage drop on the relay to the battery voltage. It hasalready been shown that the required normal ampere-turns can be obtainedwithout any increase in current density in the wire or wattagedissipation of the relay, for a given wound area, so that there is thusno limit to the degree of reduction of the operate time by reducing thenumber of turns and adding series resistance, apart from mechanicalconsiderations, such as that the minimum number of turns is one andlimitations set by the additional watts drawn from the battery andabsorbed by the series resistance. A specific embodiment will now bedescribed incorporating the above statements.

Referring to Fig. 1 a relay B of normal type is fully wound so as toproduce the ampereturns normally required, with a voltage drop whoseratio to the battery voltage Eb is the same a the required ratio ofreduction in the operate time (i. e., the time constant of the relayalone as compared with the time constant of the relay with the seriesresistance). A resistance Rs is connected in series with relay B, thebattery Eb and the operating contact Al, and absorbs the diiferencevoltage. Connected across the relay is a shunt circuit comprising afurther resistance R7 and a non-linear device MR, the latter of whichonly becomes effective in completing the shunt circuit when theoperating contact-opens. The non-linear device MR may be replaced by afurther contact A2, as illustrated in Fig, 1A, which closes when theoperating contact AI opens and vice versa, and is conveniently anadditional contact on the line relay A. In Fig. 1 rectifier MR isconnected so as to be non-conducting to the operating voltage applied tothe relay, but it automatically becomes conducting when the operatingcontact opens by virtue of the back E. M. F. generated by the decay offlux in therelay. In both cases the time constant of the decay may becontrolled, up to the maximum permitted by the relay itself, byalteration of the shunt resistance ET. This permits of adjustment of therelease lag without alteration of the mechanical adjustments. If acontact such as A2 is used instead of MR and both Al and A2 are contactsof the same relay, it is readily possible to employ a single make andbreak contact arrangement as will be readily apparent to those versed inthe art.

It should be noted that although the above described method of reducingthe operate time for given ampere-turns increases the watts drawn fromthe bat ery by comparison with a similarly wound relay giving the sameampere-turns operated directly from the battery, the watts required forthe relay itself are less than that required by a normal slow releaserelay having a shortcircuited winding. This is because the wattsreqtured are inversely proportional to the wound area for a given valueof ampere-turns, and in the above method the relay is fully wound ascompared to the present partially wound relay with an auxiliaryshort-circuited winding.

It is well-known that slow release relays of the type having ashort-circuited winding or slug sometimes fail to remain operatedor failto 0perate when the operating contact has an impulsing role, as it doesin automatic telephone systems. Both types of failure are due to thesame cause, in that the buildup flux time constant is at least as greatas the decay flux time constant release; indeed it can be shown that thebuildup time constant is the sum of the time constants of the operatingand short-circuited coils, whereas the decay, on disconnection of thebattery, has a time constant due only to the short-circuited winding.Thus, after a certain number of impulses a steady state of successiveincrements and decrements of flux is set up, whose mean value isproportional to the mean Value of the input impulses, i. e., the makepercentage of the operating contact. It follows that release of such arelay will be governed rather by the minimum make percentage of thecontact than by the break time period. Similarly failure to operate canoccur if the make percentage is too small, or at least failure tooperate over the first few impulses. Both types of failure can beavoided from the point of view of the system as a whole, if the steadystate flux on operation of the contact is attained in a time less thanthe minimum necessary for operating (or release) of the remaining relaysor magnets in the system. It is then only necessary that the release lagof the relay shall exceed the longest permissible break period, duringimpulsing.

Although adequate reduction in the time constant of the relay circuit bythe method given above will, if carried far enough, enable the steadystate flux to be substantially attained upon closure of the relaycircuit in the desired minimum time, it will be seen that if the wordsubstantially is defined as 95%, then the time constant of the circuitmust be not greater than one third of this desired minimum time, (sincethe current in a relay attains 95% of the steady state in a time equalto three times the time constant of the relay circuit). thoughattainment of the steady state to this order is very desirable, thisprocess would be very uneconomical in battery drain. If however thesteady state could be attained substantially in a period equal to thecircuit time constant, then reduction of the latter as described abovewould be both beneflcial and economical. It can be shown that theconnection of a condenser Cs as shown in Fig. 2 across the seriesresistance Rs has this effect and this addition to the embodiment beforedescribed forms the preferred embodiment of the inventlon.

It can be shown theoretically that the time to attain the steady statevalue (i. e. independently of whether the subsequent relay currentwaveform is oscillatory about the steady state value, or tends theretowithout oscillation), is equal to the time constant of the relayinductance and the series resistance, if the time constant of thecondenser with the series resistance is made equal to that of the relayalone. In general the time required to attain the steady state valuetends towards the time constant of the relay circuit without thecondenser, as the value of the latter, and the ratio between the seriesresistance and the relay resistance, increases. In order therefore toattain the steady state value of relay current within a timesubstantially equal to the time constant of the relay circuit withoutthe condenser, it is necessary that the latter shall be large and theratio between the series resistance and relay resistance shall be large.

In practice it is found that if the ratio between the series resistanceand the relay resistance is not less than 25:1, and the time constant ofthe condenser shunting the series resistance with the latter is of thesame order as the relay alone, then the desired effect is obtained. Thisvalue of condenser/series resistance time constant also gives areasonable amount of overshoot in the current waveform, which has beenfound useful in ensuring that the release flux/time waveform ismaintained even when the circuit closure time approaches the designminimum. A typical relay circuit design using a well-known type oftelephone relay normally operated on 50 volts, consists of fully windingthe relay to a resistance of ohms, whilst the series resistance has avalue of 400 ohms and the condenser shunting the latter a value of 40microfarads. The minimum time to ensure attainment of the steady stateflux with this arrangement, is less than 10 milliseconds, which is asatisfactorily low value for automatic telephone systems and will ensurethat breakdown is not now due to this relay on impulsing. The releasecircuit is of course as described iormerly, for instance, a rectifierconnected across the relay in such a manner as to be operative only tothe back E. M. on release, and of course the release lags obtainable inthese circumstances are normal since the relay is fully wound.

Provided the condenser is large enough to give a time constant with theseries resistance which is much greater than that of the relay circuitwithout the condenser (i. e., the reduced time constant due to theaddition of the series resistance), theory shows that the precise valueof the con denser is immaterial, and this is borne out in practice.Using the design quoted above, it was found permissible to alter thevalue of the condenser between 20 and 100 microfarads without materiallyaltering the time to attain the steady state current in the relay. Theovershoot of current of course, was greatly altered, being negligiblefor the lowest value and increasing for higher values. The circuit ofthis embodiment of the invention is therefore amenable to the use oflarge commercial tolerances on the condenser value, so that anelectrolytic type can be employed, whilst the possibility that the valuemay be sometimes large enough to produce considerable overshoot is of nodisadvantage in automatic telephone systems, since this only means thatthe release lag becomes larger as the operating pulses approach thespecified minimum time.

The value of the condenser should not, of course, be so large that thevoltage on it due to a previous operation of the contact has notappreciably fallen, when the next operation ensues. For instance, if thecondenser were of infinite value it follows that a subsequent operationwould only build up the relay current at a rate dependent on the relay,time constant. However this can readily be avoided by making the timeconstant of the condenser with the series resistance substantially thesame as that of the relay itself. when the condenser and relay currentswill decay at the same rates and from the same values on the opening ofthe operating contact. As was shown before this is also a suitablecondition for obtaining the steady state value upon closure of thecircuit in the minimum time period.

In applying the invention to present equipment it will be seen that therelay circuit may replace any present slow release type which has onlyone operating winding, since by using a rectifier to short-circuit therelay during release, only one operating contact is required. In thecase where it is required to hold the relay by a further contact orcontacts, without also holding or operating other relays also connectedto the original operating contact, this can be arranged by inserting anadditional rectifier in the lead from the latter to the present relaycircuit and connecting the further holding contact directly to the relaycircuit. If the rectifier be connected so as to pass current from thenormal operating contact to the present relay circuit, and other relaysenergised also from the operating contact be directly connected thereto,then when the latter contact is opened no current can flow from theholding contact for the present relay circuit to these other relays,because to do so it would have to fiow through the rectifier in thereverse direction. Such an arrangement is illustrated in Fig. 3 whichshows a preferred embodiment with the addition of a series rectifierMR2, which permits of the holding of relay B by an additional contactsuch as H, whilst not interfering with the separate operation of otherrelays such as X from the main energising contact Al. Rectifier MR2prevents the flow of current from earth through contacts H to relay Xthus causing relay X to be deenergized when contact Al opens, so that Xmay be a repeating impulsing relay operated by A.

In addition to allowing holding of relay B, rectifier MR2 also permitsthe pie-operation of relay B by a contact such as H, without operatingX. This is illustrated in Fig. 4, which shOWs the application to a Crelay in the well-known automatic telephone selector circuit. The figurealso shows the use of the invention in connection with the normal slowrelease B relay, and other relevant parts of the selector circuit.

In Fig. 4, relay A is the line relay which is controlled by a callingdevice at a subscrlbers station over lines LI and L2 in the usualmanner. Connected to the make contact of relay A is the B relay circuitaccording to the invention as disclosed in Fig. 2. Connected to thebreak contact of relay A in series with contacts B! and Cl is the Crelay circuit and stepping magnets in accordance with th invention asdisclosed in Fig. 3. A pre-operating circuit for relay C is provided inseries with the vertical oiT normal contacts VON and rotary off normalcontacts RON of the selector switch in the same manner as with contactsH in Fig. 3 although in Fig. 4 the preoperating circuit is controlled bycontacts on relay A rather than by a separate relay. The selector switchis seized by the closure of lines LI and L2 which causes the operationof relay A. Relay A operates, closes a circuit to relay B, and closes acircuit to relay C through the VON contacts. Relays B and C operate andprepare a circuit to the vertical magnet at contacts Bi and Cl.Rectifier MR2 is poled so as to prevent the operation of the verticalmagnet V at this time. Operation of the calling device at thesubscribers station interrupts the circuit to relay A a number of timescorresponding to the first digit of the called subscribers number. Eachtime that relay A restores it opens the circuit to relay B at its makecontact, and closes a circuit to the vertical magnet V and to relay Cthrough rectifier MR2 at its break contact. The first operation of thevertical magnet causes the operation of the VON contacts which remainoperated for the duration ot the call. Each time that relay A operatesduring the series of impulses it closes the circuit to relay B at itsmake contact and opens the circuit to the vertical magnet and to relay Cat its break contact. Relays B and C both remain operated during thseries of impulses due to the slugging effect of rectifiers MR and MRI.At the end of the first series of impulses relay C restores and closes acircuit to relay E from ground through operated contacts A3. operatedcontacts VON, contacts C2, and contacts E3 to relay E. Relay E operates,closes its locking circuit to contacts B2 and opens its operatingcircuit at contacts E3, transfers the pulsing circuit from the verticalmagnet V to the rotary magnet R at contacts El, and closes a circuit torelay C at contacts E2 from ground at operated contacts A3 through therotary oii normal contacts RON. This latter circuit does not affect therotary magnet R due to the uni-directional conductivity of rectifierMR2. Relay C again operates and prepares a circuit to the rotary magnetR at contacts CI. The second series of impulses causes the rotary magnetR to be operated one step for each impulse in the same manner as for thevertical magnet during the first series of impulses. The first operationof the rotary magnet R causes the operation of the rotary oii normalsprings RON which remain operated for the duration of the call. Relay Crestores at the end of the second series of impulses and opens itsholding circuit at contacts Cl. Since relay C cannot be re-operated theopening of contact Cl prevents any further operation of the steppingmagnets. When the circuit to relay A is opened at the end of the call itrestores and opens the circuit to relay B which restores and opens thelocking circuit to relay E. The restoration of relay B also closes acircuit to a release magnet (not shown) which allows the selector switchto restore to normal.

The circuit also possesses the advantage that the C relay and thestepping magnets operate in independent circuits, which makes forgreater efiiciency. The circuit of the invention is also independent ofthe back E. M. F.s of the other relays or magnets connected in parallel,since in the case of Fig. 4, MR2 takes the back E. M. F. of the magnetsin its non-conducting sense, whilst the back E. M. F. of the slowrelease relay is absorbed within itself owing to the conducting shuntrectifier and produces the slow decay time constant.

The invention may, of course, be applied to equipment other thanautomatic telephone sys tems, and where the release lag of a relay isdesired to be independent of the operating time down to a specifiedminimum value which is much less than the desired value of release lag.

With regard to the design and use of the rectifier which automaticallyshort circuits the relay winding when the contact opens. it might bethought that some difficulty would result from the reduction of theresistance of the relay. This is not so. and in fact the ratio of theforward resistance of the rectifier to that of the relay can be keptconstant at a value of about 10%, so that no material reduction of themaximum decay time constant is suffered. Since the watts in the relayconstant for given ampere-tums, it follows that as the voltage on therelay is reduced by the addition of series resistance, the current iscorrespondingly increased. Now the initial value of the decay currentequals the steady state value, and hence the cross-sectional area of therectifier must be increased with the current. Hence as the voltage onthe relay falls owing to the lower relay resistance, the number ofelements of the rectifier required to withstand this voltage also fal s,and the area of the element increases with the increase in relaycurrent, so that the forward resistance of the rectifier falls at thesame rate as the relay resistance, and their ratio is constant.

I claim:

1. In combination, a relay having an energizing winding, a rectifier, asource of direct current, a first circuit path including said winding,said rectifier, and said source connected in series a second circuitpath including said winding and said source connected in series butexcluding said rectifier, means for completing said second circuit pathto operate said relay, means for intermittently completing said firstcircuit path to maintain said relay operated, and electromagnetic meansbridging said rectifier and said relay winding operated in response tothe completion of said first circuit path to disable said second circuitpath, said rectifier preventing operation of said electromagnetic meansin response to the completion of said second circuit path.

2. A combination as claimed in claim 1 including a second relay operatedin series with said first means and contacts on said first relay inresponse to the restoration thereof upon the termination of theintermittent completion of said first circuit path for again completingsaid second circuit path.

3. In a switching system, an impulsing relay, a second relay controlledby make contacts on said impulsing relay, a rectifier connected inparallel with said second relay to retard its release, a third relay, asecond rectifier connected in parallel with said third relay to retardits release, a third rectifier, a circuit path for operating said thirdrelay from said make contacts prior to impulsing, a second circuit pathfor holding said third relay during impulsing comprising break contactson said impulsing relay, make contacts on said second relay, makecontacts on said third relay, and said third rectifier, a steppingmagnet connected to said third rectifier so as to be controlled by saidbreak contacts, and means for increasing the ratio between the meanvalue during impulsing and the steady state value of the magnetic fluxin said second and third relays comprising a parallel combination of acondenser and a resistor connected in series with the operating circuitsof said second and third relays.

4. In a switching system; an impulsing relay; a first slow release relaycontrolled by make contacts on said impulsing relay; a second slowrelease relay; a circuit path for operating said second slow releaserelay from said make contacts prior to impulsing; a second circuit pathfor holding said second slow release relay during impulsing comprisingbreak contacts on said impulsing relay, make contacts on said first slowrelease relay, make contacts on said second slow release relay, and arectifier; and a stepping magnet connected to said rectifier so as to becontrolled by said break contacts.

5. In combination, a relay having an energizing winding, a rectifier, asource of direct current, a first circuit path including said winding,said rectifier, and said source connected in series, a second circuitpath including said winding and said source connected in series butexcluding said rectifier, means for completing said second circuit pathto operate said relay, means for completing said first circuit path tomaintain said relay operated, and electromagnetic means bridging saidrectifier and said relay Winding operated in response to the completionof said first circuit path for disabling said second circuit path, saidrectifier preventing the completion of said second circuit path fromoperating said last means.

BERIRAM MORTON HADFIELD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,742,367 Nettleton et al Jan.'7, 1930 2,182,637 Marbury Dec. 5, 1939 2,279,849 Van C. Warrington Apr.14, 1942 2,299,941 Townsend Oct. 27, 1942 2,001,494 Jones May 14, 19352,128,063 Peters July 5, 1938 1,693,124 Stehlik Nov. 27, 1928 1,758,255Hudd May 13, 1930 FOREIGN PATENTS Number Country Date 812,133 FranceJan. 27, 1937

